1. Technical Field
The present invention relates to an initial focusing mechanism for a zoom lens system used to achieve initial focusing with various lens such as a variator, a compensator and focusing lenses having been assembled to a barrel so that a focal distance may be reliably varied by zooming with a given optical relationship among the various lenses being maintained.
2. Prior Art
The zoom lens system is generally used to vary a focal distance in a continuous manner and, for this purpose, is adapted to move the photographic lenses including the variator lens and the compensator lens along an optical axis with a given optical relationship among these lenses being maintained. To maintain the given optical relationship, the barrel holding these photographing lenses is formed with cam tracks or the like so that the photographing lenses are supported by the cam tracks of the barrel and movement of the photographing lenses is guided by the cam tracks.
In the course of zooming, the respective photographing lenses must be kept at given positions along the associated cam tracks and, to this end, the initial focusing is performed with the photographing lenses having been assembled to the barrel. More specifically, the focal distance adjustment is carried out with the respective photographing lenses being moved to their positions corresponding to the maximum focal distance and then with the respective photographing lenses being moved to their positions corresponding to the minimum focal distance of the zoom lens system. The typical initial focusing mechanism of prior art for zoom lens system is shown by FIG. 8 of the accompanying drawing. Referring to this figure which is a sectional view of the zoom lens system taken in the plane including an optical axis S, a barrel 1 is provided on a front end with a barrel section 2 adapted to be axially movable by a thread 2a for focusing, such as a helicoid and this movable barrel section 2 has initial focusing mechanism 3a comprising a screw or a cam by which the movable barrel section 2 supports a focusing lens frame 3 adapted, in turn, to hold a focusing lens 4.
The barrel 1 contains therein a cylinder cam 5 being rotatable with respect to the barrel 1 and this cylinder cam 5 is formed with cam tracks 5a, 5b adapted to support a variator frame 6a which holds a variator 6 and a compensator frame 7a which holds a compensator 7, respectively. Thus, rotation of the cylinder cam 5 causes the variator 6 and the compensator 7 to be guided by the cam tracks 5a, 5b, respectively, so that the variator 6 and the compensator 7 are moved along the optical axis with a given optical relationship between them being maintained. Such movement results in a change in the focal distance.
The barrel 1 is further provided on a rear end thereof with a relay lens frame 8a adapted to hold a relay lens 8. There is provided an eccentric pin 9 for initial focusing which extends through a wall of the barrel 1 and is supported rotatably with respect to the barrel 1. The eccentric pin 9 includes a pin section 9a in engagement with the relay lens frame 8a. This pin section 9a is in eccentric relationship with the rotational center of the eccentric pin 9 so that the pin section 9a swivels around the rotational center as the head section 9b of the eccentric pin 9 is rotated.
The initial focusing by use of this well known mechanism begins with movement of the variator 6, the compensator 7 and the focusing lens 4 to respective positions corresponding to the maximum focal distance of the zoom lens system. Then the initial focusing mechanism 3a associated with focusing lens 4 is adjusted and thereby the focusing lens 4 is moved along the optical axis until the focusing at the maximum focal distance is achieved. After the initial focusing mechanism 3a is locked, the variator 6, the compensator 7 and the focusing lens 4 are moved to respective positions corresponding to the minimum focal distance of the zoom lens system. From this state, the eccentric pin 9 is rotated to move the relay lens 8 along the optical axis until focusing at the minimum focal distance is achieved, then the eccentric pin 9 is locked in order to lock the relay lens 8 and thereby the composite initial focusing is completed.
However, with the initial focusing mechanism of the prior art as has been mentioned above, the initial focusing adaptor 3a and the thread 2a are provided in operative association with the focusing lens 4, resulting in that the barrel portion carrying the focusing lens 4 inclusive of adjustment devices 2a , 3a inconveniently has a relatively large outer diameter.
While it is preferred to minimize the outer diameter of the zoom lens system for improvement of the camera the characteristics such as maneuverability and portability, the outer diameter of the zoom lens system necessarily becomes larger than the diameter of the focusing lens 4 due to the presence of mechanisms 2a, 3a.
As has been described above, the initial focusing mechanism of prior art achieves the initial focusing by moving the focusing lens 4 along the optical axis with respect to the variator 6 and the compensator 7. In other words, the conventional initial focusing mechanism generally relies on a variation of the relative position or distance of the focusing lens 4 to the variator 6 and compensator 7.
In view of the fact that the initial focusing can be achieved by varying the relative position or distance of the focusing lens to the variator and the compensator, it is a principal object of the present invention to provide an improved initial focusing mechanism allowing a desired initial focusing to be achieved without incorporation of any initial focusing devices into the focusing lens system and thereby allowing the zoom lens to be compactly configured.